1. Field of the Invention
The present invention relates to a method using sol-gel chemistry for making transparent, optical quality glass having a selected bulk refractive index.
2. Description of Related Art
Numerous methods have been developed for making porous solids having open pores, e.g., polymer foams, pre-ceramics, and porous glasses, including aerogels and xerogels. Aerogels and xerogels are of particular interest because of their nanometer-size pores, which are smaller than the wavelengths of visible light, resulting in transparency and other exceptional properties. U.S. Pat. Nos. 5,275,796, 5,409,683, and 5,686,031 describe processes for making inorganic aerogels and xerogels and other porous materials. U.S. Pat. No. 5,076,980 also discloses a method for making sol-gel monoliths.
Porous glasses can be used in a wide variety of optical applications, such as for graded refractive index lenses, waveguides, fiber optics, and high energy particle detectors. In these applications, controlling the index of refraction of the material is of particular interest. U.S. Pat. No. 5,047,776, for example, describes a Luneburg lens made of concentric dielectric shells of aerogel material having various indices of refraction. U.S. Pat. No. 5,684,907 describes an optical fiber surrounded by an aerogel cladding, whose index of refraction is close to that of air.
The index of refraction (n) of a porous glass is a function of the bulk density (.rho.) of the material: n=1+k.rho.. The constant k can be determined for a specific material from its full density values of n and .rho.; i.e., k=(n.sub.s -1)/.rho..sub.s. For example, silica (SiO.sub.2) has a bulk density of 2.2 g/cm.sup.3 and an index of refraction of 1.46, so k has a value of 0.21 cm.sup.3 /g. Thus, in order to manufacture a porous glass with a specified refractive index, the density must be controlled. The density (or porosity) of the final product is determined by the concentration of initial reactants and the method used to dry the material.
U.S. Pat. Nos. 5,275,796, 5,409,683, and 5,686,031 (cited above) describe the formation of porous glasses using sol-gel chemistry using a precursor solution that is gelled and then dried by removing the liquid from a two-phase liquid-solid network. Ultralightweight materials, such as aerogels, are typically dried by extracting the liquid under supercritical conditions. Denser materials, called xerogels, are formed by evaporating the liquid under ambient conditions, which causes significant shrinkage (collapse) of the solid network. These methods require a separate solution for each different density material with reactants in specified proportions. These methods are further limited by not producing a full, uninterrupted range of possible densities (or refractive indices) while maintaining optical transparency.
This limitation is a result of the slow evaporation of imbibed liquid during the production of xerogels, which typically causes a structural rearrangement of the particles within the gel as it shrinks. At some point in this process, the particles within the gel begin to scatter the light passing through it and the gel becomes opaque, rather than fully transmitting light so that the gel is translucent. The gel is not fully dried when this occurs, so shrinkage normally continues and the density increases until the gel is dry. Thus, normal porous glass processing by evaporation to dryness cannot achieve certain densities in the range (for silica) less than 1.1 g/cm.sup.3 (corresponding to a refractive index less than 1.23), nor can they produce transparent glass in this range.
The present invention addresses these limitations by providing a method that can produce optically transparent glasses with any specified index of refraction, particularly in the range of 1.05-1.28, and additionally, can produce glasses having a range of varying densities using a stock solution, rather than separate solutions for different glass densities.